Environmental control provides significant benefits when applied to production cycles of chickens and other species and varieties of animals and plants. Whilst increased mortality rates result from extreme environmental conditions, optimal environmental conditions positively impact on growth and general well being. For example, feed conversion and weight gain ratios can be improved through environmental control.
Three main inter-related environmental parameters have a significant influence in a chicken house, namely temperature, humidity and wind velocity. Specifically, links exist between temperature, humidity, and bird age or weight in terms of heat stress and between wind velocity, temperature and bird age or weight in terms of wind chill.
The temperature that the birds actually experience as opposed to the temperature directly measured within a chicken house is known as a perceived temperature. Generally, chickens feel warmer as the humidity increases and cooler as the wind velocity increases. Age, and in particular bird weight, also has a direct bearing on the effects of these parameters. For example, young chicks are sensitive to wind chill and are easily over-cooled if exposed to wind. Stress resulting from over-cooling may result in reduced weight gain throughout a production cycle (nominally 5 to 8 weeks).
Conventional environmental control systems use measured temperature as an input to a feedback control loop to control or regulate ventilation cooling in a chicken house. Chicken houses are generally cooled by the activation and de-activation of fans that blow air down the length or across the width of the house. The wind cools the birds by way of a wind chill effect. Air may also be brought in through pads which have been sprayed with water to further aid cooling in an evaporative cooling process.
Whilst such rudimentary control may be capable of maintaining temperature levels within certain limits and influencing humidity levels to a limited degree, effective humidity control requires capital-intensive air conditioning equipment and is thus not generally used in the agricultural industry.
There exists a standard heat stress equation for human comfort levels that describes the relationship between temperature (dry bulb), Heat Index (HI) and humidity. FIG. 4 shows a graph of HI curves as a function of temperature and humidity levels, as well as the locus of points indicating the points at which HI=T.
The U.S. Weather Services uses the standard equation for Heat Index (HI), with constants adapted for human comfort levels, in the following form:
  HI  =            -      42.379        +          2.04901523      ⁢      T        +          10.14333127      ⁢      H        -          0.22475541      ⁢      TH        -          6.83783      ×              10                  -          3                    ⁢              T        2              -          5.481717      ×              10                  -          2                    ⁢              H        2              +          1.22874      ×              10                  -          3                    ⁢              T        2            ⁢      H        +          8.5282      ×              10        4            ⁢              TH        2              -          1.99      ×              10                  -          6                    ⁢              T        2            ⁢              H        2            
where:                T=ambient dry bulb temperature in ° F., and        H=relative humidity (%)        
A Heat Index of greater than 130° F. (54.4° C.) is indicative of conditions that are extremely dangerous for human beings.
Whilst it is desirable to automate control of environmental parameters in a feedback control loop, a difficulty arises in unifying the parameters in a single solution due to the complexity of the standard heat stress equation. Further, the standard equation is unstable in that small variations to constants in the equation result in large variations in output of the equation. A need thus exists for an improved method and apparatus for controlling environmental parameters of temperature, relative humidity and wind chill. Another need exists for an improved method and apparatus for controlling environmental parameters in a chicken house.